Biomedical Engineering Reference
In-Depth Information
Another division of the warfare arsenal against foreign intruders as discussed
thus far is the
complement system
. A one-liner saying states that 'antibodies iden-
tify targets, but the complement system is destroying them'. It is an intrinsic and
important part of the innate immune system. The definition is that the complement
system is a biochemical cascade, which helps to clear pathogens from an organ-
ism. Inappropriate activation of the complement system by endoprosthetic devices
is described for cellulosic Cuprophan membranes used in hemodialysis and for some
other extracorporeal devices. The complement system will of course be activated by
tissue- or collagen-based materials harvested in insufficiently related species, e.g.,
porcine heart valves. For further reading, see [
139
]and[
140
], pp. 293-354.
4.4
Gulliver and the Lilliputians
The action of macrophages, antibodies and other defense weaponry of foregoing
sections can be understood as long as cell-sized objects such as bacteria or metal
ions are put on the stage. To be experienced by the poor little tissue cells, however,
the giant body of the implant is above their individual abilities: Lilliputians looking
at the sleeping Gulliver. When Gulliver woke up on the beach, he found himself tied
up by the collective effort of the 15 cm-tall Lilliputians ...a worst case scenario for
an implant: tied up by collagenous ropes.
Bacteria and small foreign bodies can be phagocytized, implants cannot, a matter
of scale! Comprehensive treatment of host reactions to the wide variety of bioma-
terials for soft and hard tissues (cardiovascular, orthopedic, dental, etc.) would be
a precarious enterprise. Therefore, the following report is restrained to a diagonal
reading through a sequence of events happening to a hip stem implanted in a femur
and the response of the injured tissue. The sequence is schematized in Fig.
4.3
.
The first minutes to days.
The surgical access to the proximal part of the femur
through skin and muscles, removal of the head and reaming the femoral canal elicits
an immediate tissue response:
acute inflammation
. Extravasation of plasma proteins
and fluid causes swelling, ((o)edema). Simultaneously, polymorphonuclear leuko-
cytes (PMNs) are migrating to the site of the trauma.
10
The driving force is said to
be a chemical gradient (
chemotaxis
). The implant surface is recognized through
serum factors, called
opsonins
, adsorbed on the surface. As stated above, phagocy-
tosis of the implant is not possible but
frustrated phagocytosis
may occur: release
of leukocyte products, which may degrade the material. Thioneins also become
immediately part of the emergency battery, secreted at the site by PMN and other
cells hurrying to the place of calamity.
<
10
It is an alternative term for a category of leukocytes or white blood cells, namely
granulocytes
.
They all possess irregular multilobed nuclei, from where the term derived. Another category of
leukocytes are the
mononuclear phagocytes
or
monocytes
.
